1. Field of the Invention
The present invention relates to a switching configuration for telecommunication exchange installations, of the PCM time division multiplex type having a central switching network and individual sub-switching networks connected thereto.
2. Description of the Prior Art
Through the Journal "telecom report", Vol. IV (1981), Supplement "Digitalvermittlungs system EWSD" (EWSD digital exchange system), at page 19, PCM telephone exchange systems are known which are equipped with a multistage central processor having time division multiplex inputs and time division multiplex outputs as well as with a plurality of line groups functioning to connect time division multiplex interexchange trunks of which each is connected to a time division multiplex input and a time division multiplex output of the switching network, and of which each has a plurality of line units functioning to connect in each instance one time division multiplex interexchange trunk, which, in turn, is connected to a time division multiplex switching system associated with the particular line group and can be connected through it to channels of the time division multiplex input corresponding to the particular line groups and the corresponding time division multiplex output of the switching network.
In telephone exchange systems of this kind time division multiplex interexchange trunks always combine a large number of channel pairs, for example twenty-four, thirty, sixty or even more. A channel pair always comprises a channel in the one transmission direction and a channel in the other transmission direction. One channel pair is required for each connection, aside from multi-channel connections. As the illustration in the journal referred to above shows, a large number of line groups for each telephone exchange is provided with a switching network which, to be on the safe side, is duplicated. Each line group comprises a plurality of line units. There are line units of different kinds depending on the specific kind of the connected lines. Subscriber lines are, as a rule, analog lines, but can also be realized in PCM technology. Interexchange trunks can also be analog lines. They can, however, also be realized as PCM channels which, in known manner, are combined in PCM transmission systems with one time division multiplex line each. The mentioned line units are constructed differently and, specifically, in each instance corresponding to the particular technology of the subscriber line and interexchange trunks to be connected and depending on whether or not analog technology or time division multiplex technology is utilized. As a rule, a line system is provided for a plurality of analog subscriber lines, for example eight. Similarly, one line unit is provided for a plurality of analog interexchange trunks. For each time division multiplex interexchange trunk of a PCM transmission system, as a rule, one line system is provided. But it would be equally possible to connect to one single line system more than one time division multiplex interexchange trunk of a PCM transmission system, for example two time division multiplex lines.
Through the natural combination of the number of channels described hereinafter in one single PCM transmission system, its assignment is obviously also limited to only one single direction because the particular channels must run the same way as the time division multiplex lines of this PCM transmission system. Since, likewise, due to this combination technique, the connection of a time division multiplex line of a PCM transmission system to a branch line unit, by necessity, also results in an assignment of the particular channels to a single line unit and, hence, to a single line group, in the event that for one direction only the channels (channel pairs of a single PCM transmission system are available upon breakdown of a line group all possibilities for connections in the particular direction fail and also, from the aspect of switching, a whole trunk group fails.
In order to avoid the previously described total failure of a trunk group due solely to the failure of a line group, until now in the prior art it was known to provide independently of each other two PCM transmission systems for each direction, and to connect the time division multiplex lines of this transmission system to line units of two different line groups. This is costly and uneconomical, particularly with small trunk groups due to duplication of the PCM transmission system.
In order not to have to provide in a switching configuration of the initially indicated kind in view of the possibility of failure of a line group duplication of a PCM transmission system for each direction, and in order to avoid in so doing that due to the failure of a line group alone, i.e. The failure of the channel pairs of the particular PCM transmission system, a whole direction can fail respectively must, for a telephone switching system of the initially indicated known kind according to Luxemburg Patent No. 86,734 (corresponds in terms of content to the European Patent application No. 87 108 702.9) it has been provided to assign two line groups pairwise to each other and it is, furthermore, provided that the line systems of each of these two line groups which, during normal operation are connected to the time division multiplex switching system of their own line group, can be switched over during emergency operation of one of the two line groups to the time division multiplex switching system of the other line group.
A PCM transmission system with its channels in the event of failure of the particular line group can continue to be operated. In this operation, a duplicate number of channels (channel pairs) is switched via a line group, i.e. via the other of the two line groups assigned to each other in pairs. This does, lead to a particularly heavy traffic load of the particular line group and, possibility to an increase of path busy occurrences, however, all connections in the process of being set up via the particular channels have the same chances of succeeding. It can happen that a plurality of directions can only be operated with half the traffic handling capacity. However, connections in all traffic directions can still be established and the total failure of the primarily affected traffic direction respectively traffic directions is prevented. By stronger proportioning of the sub switching networks of each of the line groups with respect to the number of the individual through-connection possibilities as well as with respect to the processing capacity of each of their individual control units the traffic handling capacity can, of course, again be increased in both respects.